If you overdosed on probiotics you could have SIBO.
It doesn't matter if the bacteria are good or bad - if they start growing in your small intestine they will make you sick. The small intestine is supposed to be near sterile. Maybe ask your doctor about a SIBO breath test?
I've also had chronic nausea and now I believe it was partly caused by low stomach acid. You could try taking peppermint or bitter herbs before eating, or raw ACV (must be raw for enzymes) with meals.